Engines may be configured to deliver fuel to an engine cylinder using one or more of port and direct injection. Port fuel direct injection (PFDI) engines are capable of leveraging both fuel injection systems. For example, at high engine loads, fuel may be directly injected into an engine cylinder via a direct injector, thereby leveraging the charge cooling properties of the direct injection (DI). At lower engine loads and at engine starts, fuel may be injected into an intake port of the engine cylinder via a port fuel injector, reducing particulate matter emissions. During still other conditions, a portion of fuel may be delivered to the cylinder via the port injector while a remainder of the fuel is delivered to the cylinder via the direct injector.
During periods of engine operation where direct injection of fuel is disabled and no fuel is being released by the direct injector (e.g., during conditions where only port injection of fuel is scheduled), fuel trapped inside the DI fuel rail may expand due to high temperatures. This can result in a pressure build-up in the DI fuel rail as well as elevated injector tip temperatures. If the deactivation period of the DI is long, the pressure built up may be significant. Prolonged exposure to such high pressure conditions may cause damage to the fuel system components. To address this, while direct injection is disabled, a small amount of fuel may be intermittently released from the direct injectors in order to bleed-off the excess pressure in the direct injection fuel rail and lower the injector tip temperature.
However, the inventors have identified potential issues with the above mentioned approach. As one example, activation of the direct injectors for DI fuel rail pressure relief generates a high impact force that is transmitted from the injectors onto the engine cylinder heads. This produces a ticking noise in the vehicle that may be objectionable to the vehicle operator. As such, the higher the rail pressure, the louder the ticking noise that is generated. Additionally, if the port injection is being used during engine idling, where the engine noise is low, there may not be sufficient engine noise to mask the ticking noise, making the ticking noise more audible and objectionable to the operator. In addition, the high pressure imparted by the direct injected fuel on the cylinder heads can damage the cylinder head, resulting in warranty issues.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: during an engine warm idling condition, maintaining direct injectors disabled until a direct injection fuel rail pressure is reduced via a high pressure pump relief valve; and then further reducing the direct injection fuel rail pressure via intermittent activation of the direct injectors. In this way, the pressure at the direct injection fuel rail may be relieved with reduced ticking noise.
As an example, an engine may be configured with each of port and direct fuel injection. Fuel may be delivered to the port injection fuel rail via a low pressure lift pump. Pressurized fuel may then be delivered to the direct injection fuel rail via a high pressure pump (HPP) receiving fuel from the low pressure lift pump (LPP). During warm idling conditions, fuel may be delivered to the engine via port injection only and the direct injectors may be disabled. Consequently, pressure may build up from fuel trapped at the direct injection fuel rail, resulting in an elevated pressure being experienced at the HPP. A controller may determine an amount of pressure relief required based at least on a duration of direct injector deactivation (or a duration of PFI only operation), and further based on engine operating conditions. The DI rail pressure may then be relieved while maintaining the direct injectors disabled. As a first step, as the rail pressure exceeds a first threshold pressure that corresponds to a high pressure (HP) pump relief pressure, a pump relief valve coupled to the HPP may intermittently open (e.g., automatically via mechanical actuation) to maintain the rail pressure at the first threshold pressure. If further pressure relief is required, such as when a duration of DI deactivation is longer than a threshold duration, the direct injectors may be intermittently activated to deliver a small pulse of fuel into the cylinders. By relieving at least some of the pressure via the pump relief valve, the additional pressure relief required via the direct injectors (if required) may entail a smaller number of fuel pulses as well as fuel pulses of a smaller pulse-width than would have been required if only direct injection were used for pressure relief. Due to the smaller size and number of fuel pulses, as well as the lower fuel rail pressure at which the injectors are activated, the impact force transmitted from the injection onto the engine cylinder heads may be substantially lower (e.g., negligible), resulting in reduced occurrence of objectionable ticking noise. In addition, damage to fuel system components is reduced.
In this way, pressure may be relieved from a DI fuel rail and at a related HPP with less generation of objectionable noise, such as less ticking noise. By enabling DI fuel rail pressure to be reduced below a relief threshold of the HPP via operation of the pump relief valve, the direct injectors may be maintained deactivated for a longer duration, reducing the occurrence of ticking. Even when the direct injectors are activated for pressure relief, since a smaller degree of pressure relief is required via the injectors, due to the pressure relief provided via the pump relief valve, the amount of objectionable noise generated may be substantially lower, or negligible. As such, the lower volume ticking noise may be low enough to be masked by engine noise such that it is not audible (or objectionable) to the operator. Also, by reducing the impact force on the cylinder head from the direct injection, component life is extended, reducing warranty issues.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.